This invention is related to small four-cycle internal combustion engines and in particular to a compression relief mechanism to facilitate engine cranking.
Small internal combustion engines have found wide acceptance in garden implements such as line trimmers and leaf blowers and power tools such as chain saws. Initially, small two-cycle engines were used for these applications. However, two-cycle engines have well recognized exhaust emission problems that often make them unacceptable for their use in engines that must comply with exhaust emission regulations such as the California Air Resource Board and the Federal Environmental Protection Agency (xe2x80x9cEPAxe2x80x9d) regulations.
Limitations on exhaust emissions of carbon monoxide, hydrocarbons and nitrogen oxide that will be required in the near future cannot feasibly be met by outdoor power tools powered by two-cycle internal combustion engines. Four-cycle internal combustion engines in contrast provide a distinct advantage in that they are capable of meeting the new exhaust regulations and are quieter compared to a comparable two-cycle engines.
A problem currently being faced with the small four-cycle engine is the force required to crank them to start. Since there is no substantial overlap between the exhaust and fuel intake cycles of a four-cycle engine, the force required to overcome the compression cycle of the four-cycle engines becomes much higher. This problem was recognized by the prior art and various mechanisms have been disclosed to reduce the manual force required to overcome the compression stroke. For example, Yamashita, et al in U.S. Pat. No. 4,651,687; Holsehub in U.S. Pat. No. 4,977,868; Teral, et al in U.S. Pat. No. 4,991,551; and Kojima, et al in U.S. Pat. No. 5,948,992 all teach pressure release mechanisms deactivated by centrifugal force when the engine reaches operating speed. These mechanisms require moving parts and are equally actuated during the exhaust as well as the compression cycles keeping the exhaust valve partially open during the intake stroke as well.
The invention is an improved compression relief mechanism for small four-cycle engines of the type having a single cam actuating the exhaust and intake valves. The invention comprises a second cam surface provided on the single cam and either the intake valve cam follower or the exhaust valve follower has a second cam engagement surface which engages the second cam surface to partially open either the intake or the exhaust valve during the compression cycle to effect a compression relief reducing the force required to crank the engine.
A first object of the invention is to provide a compression relief mechanism having no moving parts.
Another object of the invention is to provide a compression relief mechanism for a four-cycle engine which is actuated only during the compression cycle.
Another object of the invention is to provide a second cam surface provided on single cam engageable with a second cam engagement surface on either the intake valve cam follower or the exhaust valve cam follower.
Still another object of the invention is to provide a boss extending from the side of the single cam lobe which provides the second cam surface and the cam follower has a second cam engagement surface which engages the boss to partially open either the intake or exhaust valve during a predetermined period during the compression cycle.
Yet another object of the invention is a mechanism for disabling the engagement of secondary cam engagement surface with the secondary cam surface at normal engine operating speeds.
These and other objects of the invention will become more apparent from a reading the detailed description of the preferred embodiment in conjunction with the appended drawings.